1. Field of the Invention
The present invention generally relates to inductors, and more specifically, to variable inductors.
2. Description of the Related Art
Inductance is typified by the behavior of a coil of an electrical conductor in resisting any change of electric current through the coil. Faraday's law defines inductance in terms of the amount of voltage generated to oppose a given change in current: V=−L*di/dt where L is inductance, in henries, V is voltage in volts, and di/dt is change of current in amperes per second.
The inductance of a coil comprising an electrical conductor is entirely determined by the physical properties of the coil. The electrical conductor is commonly made of wire (e.g., copper, aluminum, etc). Wire will be used hereafter to describe the conducting material, although those skilled in the art will understand that any conducting material is contemplated. Coils are also commonly called inductors. A coil has a number of loops, or turns, about an axis of the coil. As electrical current flows through the wire, a magnetic field is created around the wire. As the electrical current increases, the magnetic field increases. The magnetic field cuts other loops in the coil and has the effect of increasing the opposition to a change in current. As described above, opposition to a change in current is inductance.
Inductors may have various materials (including air) forming some or all of the volume axially inside the coil. The material chosen is called the core of the inductor. Ferrous materials (or other magnetic materials) are often used as the core of the inductor, and use of such materials dramatically increases the inductance of the inductor. Air is also frequently used as the core. Nonmagnetic materials, such as plastic, are often used to wrap the wire around, but do not substantially alter the inductance of the coil, and such coils are also generally referred to as air core inductors. Some environments require that no magnetic materials be used in the inductor. For example, variable inductors used in Magnetic Resonance Imagers (MRI) applications, must have no magnetic material in volumes sensitive to such materials. For example, “birdcage” resonators often used with MRI applications must be designed without magnetic materials. For an overview of birdcage resonators and associated circuitry, see “A Transmit-Only/Receive-Only (TORO) RF System for High-Field MRI/MRS Applications”, by Enzo A. Barberi, et al., published in Magnetic Resonance in Medicine 43:284-289 (2000), hereinafter “Barberi”. This reference can currently be read at http://www.lfmrr.rri.on.ca/research/MRM Barberi hybrid TORO.pdf on the WorldWide Web. Another background reference is “Magnetic Resonance Imaging”, by Joseph P. Hornak, Ph.D., which can currently be read at http://www.cis.rit.edu/class/schp730/lect/lect-17.htm.
There are several factors that affect the inductance of a coil. The number of turns of the coil, the diameter of the coil, the length of the coil, the number of layers of wires in the coil, and the type of material used in the core all affect the inductance of the coil, as is known by those skilled in the art. For example, if twice as many turns of wire are used (other factors being the same), the inductance will increase by a factor of four, the inductance varying as the square of the number of turns of the wiring. (The field is twice as strong and the field cuts twice as many turns). The diameter of the coil also dramatically affects the inductance of the coil. Inductance increases as the square of the cross-sectional area of the coil. The length of the coil affects the inductance as well, since, for a given number of turns of the coil, as the coils are stretched apart, fewer magnetic flux linkages exist due to the greater distance between each turn. Other factors being the same, doubling the length of a coil halves the inductance of the coil. Finally, the type of core material used affects the inductance of the coil. A ferrous (e.g., soft iron) core has a high magnetic permeability relative to an air core resulting in
more lines of magnetic force, thus increasing the inductance of the coil. Inductance is directly proportional to the permeability of the core material.
With the above brief review of inductors behind us, we turn to the problem of constructing an adjustable method of tuning a coil's inductance. Applications exist for tuning inductors in medical, industrial, and scientific application. In many ferrous (or other magnetic material) core inductors, inductance is tuned by controlling the distance of penetration of a magnetic material into the core of the inductor. For example, the portion of the core filled by the magnetic material is determined by turning a threaded screw which pushes/pulls the magnetic material along the axis of the coil. As the magnetic material comprises more of the core, inductance of the coil increases.
Tuning of an air core inductor is more difficult than tuning a magnetic material core inductor. Air core inductors are required in some environments. We are faced with the prospect of “squeezing” or pulling apart air core coils in an attempt to adjust them to their desired inductance value. Squeezing together or pulling apart the windings of an air core coil is mechanically awkward or impossible to do, depending on the position of the air core coil in the electronic system. Furthermore, if relatively high magnetic fields are created, electromechanical forces can cause the length of the coil to be shortened, especially if relatively thin wire is used in the coil.
Therefore, apparatus and methods are needed to facilitate adjustment of an inductor by altering the physical shape of the inductor and holding the inductor in the desired shape.